Make CMake official? (Makefile build does not provide CMake config file)

README.md says

> make is the officially maintained build system of this project.

When using Makefile, CMake config files like `zstdConfig.cmake` is not installed. This makes projects using CMake awkward to use zstd. E.g. llvm-project has

```cmake
// https://github.com/llvm/llvm-project/blob/main/llvm/cmake/config-ix.cmake
if(LLVM_ENABLE_ZSTD)
if(LLVM_ENABLE_ZSTD STREQUAL FORCE_ON)
find_package(zstd REQUIRED)
if(NOT zstd_FOUND)
message(FATAL_ERROR "Failed to configure zstd, but LLVM_ENABLE_ZSTD is FORCE_ON")
endif()
elseif(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
find_package(zstd QUIET)
endif()
endif()
set(LLVM_ENABLE_ZSTD ${zstd_FOUND})

// https://github.com/llvm/llvm-project/blob/main/llvm/lib/Support/CMakeLists.txt#L28
if(LLVM_ENABLE_ZSTD)
if(TARGET zstd::libzstd_shared AND NOT LLVM_USE_STATIC_ZSTD)
set(zstd_target zstd::libzstd_shared)
else()
set(zstd_target zstd::libzstd_static)
endif()
endif()
```

It could add pkg-config fallback but that is inconvenient, and logic like `zstd::libzstd_shared` does not have a good replacement.

Related:

* https://bugs.gentoo.org/872254
* https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1020403

The simplest solution is to make CMake official so downstream is motivated to switch to CMake.